The present invention relates to methods for use in cyclic continuous adsorption systems for recovering solvents from solvent laden air using activated carbon in two or more adsorber vessels as an adsorbent and live steam as the desorption media wherein one or more of the adsorber vessels are taken out of adsorption service in turn for regeneration.
Various methods and apparatus for recovering solvents from solvent laden air are well known in the prior art. In the past, it has been common to use activated carbon located in two or more adsorber vessels as the adsorption medium and live steam as the desorption medium. The solvent laden air is passed through the adsorber vessels and the solvent adsorbed by the adsorption medium; when the adsorption medium is fully charged, steam is passed through the vessel to heat the adsorption medium and desorb the solvent therefrom. This desorption operation is often referred to as "scrubbing". The solvent is then removed from the adsorber vessel by the steam and the steam/solvent vapor mixture may be condensed outside of the adsorber. Continuous operation of such recovery systems has been achieved by cyclically removing one or more, but less than all, of the adsorber vessels from adsorption service and scrubbing the adsorption medium in those vessels while the remaining vessels remain available for adsorption service.
It is known in the art that following the steam scrubbing operation, the adsorption medium must be both cooled down from the relatively high temperature to which it is heated by contact with the steam during the scrubbing operation and dried by removal of most of the water vapor remaining therein prior to its again returning to efficient adsorption operation. The cooling/drying step may be performed with the solvent laden air itself using the initial portion of the solvent laden air that passes through the adsorber vessel to perform the cooling/drying. Alternatively, the cooling/drying may be performed by using a fan to draw fresh air from the atmosphere through the adsorber vessel immediately following the scrubbing stage. In either case, the atmospheric air or the solvent laden air which passes through the adsorber vessel during the cooling/drying step is vented directly to the atmosphere resulting in an undesirable peak in the vapor emission from the recovery system since at least the steam, water vapor, and any solvent contained therein which are present in the adsorption vessel at the end of the steam scrubbing operation are vented directly to the atmosphere.
Because of increasing awareness of the problems associated with venting excessively large amounts of solvent material into the air, emission limitations have been imposed by various governmental bodies. Such limitations often place restrictions on both the average and peak emission levels of a particular facility. Moreover, because of the public awareness of the problems which can be associated with excessive emission levels, it is often desirable to reduce the level of highly visible, but relatively innocuous substances such as water vapor. Thus, it has become highly desirable to find methods of reducing emissions from systems of the type described which are both highly effective and economical both in construction and operation.
One such method is the subject of patent application No. 2 222 400 of the German Federal Republic and French Pat. No. 73 15869. In that method, an intermediate or purging step is interposed between the steam scrubbing operation and the cooling/drying step. The purging step is performed by drawing fresh air into the adsorber vessel and exhausting the vapors from the adsorber to a condenser until acceptably low levels of solvent and water vapor concentrations within the adsorber vessel is obtained. The exhausted air flow is introduced into the solvent laden air flow and thus passed through the adsorbing vessels which are then in adsorption service. The fresh air flow through the adsorption medium during the purging step is in the opposite direction of the flow of solvent laden air during the adsorption operation. The cooling/drying step is then performed using either fresh air or solvent laden air as discussed above.
The method discussed has been found to have a number of disadvantages associated therewith. For one, the adsorption vessels and associated fans and other components must be somewhat oversized since they must handle the fresh air used during the purging step as well as the solvent laden air. For another, the passage of air through the adsorption medium during the purging step is in the reverse direction of the flow of solvent laden air during adsorption, a condition which is believed to be undesirable.
The purpose of this invention is to provide methods which overcome the above noted deficiencies of present solvent recovery systems. More specifically, there are provided by this invention solvent recovery methods by which a purging step is provided between the steam scrubbing and cooling/drying step which purging step makes use of solvent laden air.
It is an object of this invention to provide solvent recovery methods which improve over the methods presently extant in the manners above noted.
It is an object of this invention to provide solvent recovery methods which are characterized by both effectiveness in operation and economy in construction and operation.
It is an object of this invention to provide solvent recovery methods which are effective to reduce the peak vapor emission levels from solvent recovery systems.
Further and additional objects of this invention will be apparent to those skilled in the art from the description, the drawing, and the appended claims.